1. Field of the Invention
The present invention relates to a lens barrel using a varifocal optical system, and more particularly, to an improvement of a lens driving system for performing zooming and focusing by use of a built-in driving source such as a motor.
2. Description of the Prior Art
In a typical zoom lens barrel, to perform zooming and focusing, a plurality of lens units are divided into zooming units exclusively used for zooming and focusing units exclusively used for focusing to fix the role of each lens unit, a zoom cam and a groove for converting an operation of an operation ring into a movement of the lens units are formed on a ring or a stationary barrel for moving the lens units, and zooming and focusing are performed by a combination of the zoom cam and the groove. In this arrangement, however, the zoom ratio cannot be increased and a lens capable of focusing for near objects cannot be realized.
On the contrary, a lens barrel having a varifocal optical system realizes an increased zoom ratio and focusing for near objects by using the zooming lens units also for focusing. This type of lens barrel employs a mechanism comprising a combination of a zoom cam and a focus cam where zooming is performed by rotating the zoom cam and focusing is performed by non-rotatively moving the focus cam along the optical axis.
In this case, as shown in FIG. 1, a zoom cam z and a focus cam f are both formed to be linear in a developed condition. In this arrangement, however, the ratio between the movement amount of the cam and the movement amount of the lens is the same at the zoom position on the short focal length side (wide angle side) and at the zoom position on the long focal length side (telephoto side), so that the movement amount of the focus cam ring varies. Therefore, this arrangement cannot be put into practical use.
Thus, in the varifocal optical system, since the focusing movement amount of a predetermined lens unit from infinity with respect to a subject varies according to the zoom position, as shown in FIGS. 2A and 2B, the cam configuration is necessarily determined so that a movement amount x of the focus cam ring is fixed. With such a cam configuration, the movement amount of the focus cam ring is the same both at the wide angle side zooming position and at the telephoto side zooming position similarly to the normal zoom lens although the movement amount of the cam varies between a movement amount y1 on the wide angle side shown in FIG. 2A and a movement amount y2 on the telephoto side shown in FIG. 2B.
FIGS. 3A and 3B show a movement of a focusing block as a basic mechanism of the varifocal optical system. A zoom cam ring 51 is rotatively moved. A stationary barrel 52 is stationary. A focus cam ring 53 is moved along the optical axis. Numeral 54 represents a lens moving frame on which a guide pin 55 is formed. The guide pin 55 engages both with a cam groove of the zoom cam ring 51 and with a cam groove of the focus cam ring 53. Reference character p represents a movement amount of the second lens unit on the wide angle side during focusing. Reference character q represents a movement amount of the second lens unit on the telephoto side during focusing. Typically, q is greater than p.
FIG. 4 shows an example of a focusing mechanism provided in a conventional lens barrel having a varifocal optical system. In the lens barrel shown in the figure, a helicoid cylinder 56 having a helicoid screw 56a on its inner dimension and having a helicoid gear 56b on its outer dimension is held to be only rotatable. The helicoid gear 56a engages with a screw 57a formed on a focus cam ring 57. The screw 57a is circumferentially formed on an end portion of the focus cam ring 57. The helicoid gear 56b meshes with a gear 58a of a focus deceleration gear train 58. The input of the focus deceleration gear train 58 is coupled to a manual focus ring (not shown) and to an AF (auto-focus) motor 59.
In the above-described conventional arrangement, the rotation of a focusing ring (not shown) or the rotation driving of the AF motor 59 is decelerated by the focus deceleration gear train 58 and transmitted to the helicoid cylinder 56, and by the rotation of the helicoid cylinder 56, the focus cam ring 57 coupled to the helicoid screw 56a is non-rotatively moved.
In the above-described conventional arrangement, however, since a heavyweight gear train such as the large diameter helicoid cylinder 56 is necessary and the proportion of the deceleration by the gear deceleration system is greater as shown in FIG. 5, the size increases, so that the size and weight of the camera cannot be reduced. Since the gear deceleration system is a combination of a plurality of gears, the number of parts increases to increase the weight. In addition, since a large motor having a large torque is necessary to drive such a gear train, the cost increases.
In order to prevent over-engagement between the screw 57a of the focus cam ring 57 and the helicoid screw 56a at the drive end, it is necessary to provide, for example, a focus stopper (not shown) on the stationary barrel or on the focusing ring to limit the rotation of the focusing ring. This results in an increase in the number of parts and in the number of production processes.
To solve such problems of the lens driving mechanism using a helicoid, an arrangement using a feed screw 60 as shown in FIG. 6A has been proposed. In this lens driving mechanism, since it is necessary to prevent over-engagement of the feed screw 60 when the lens is moved to the limit position, a guide cylinder 61 for guiding the feed screw 60 has a hole 61a passing through its peripheral wall, a ball 62 is inserted in the hole 61a so as to roll along the root of the feed screw 60, and the ball 62 is pressed against the feed screw 60 by a flat spring 63 provided on the guide cylinder 61.
In this arrangement, as shown in FIG. 6B, when the feed screw 60 is further driven after reaching the drive end, the ball 62 is rolled over a non-threaded portion 60a of the feed screw 60 and pushed out of the hole 61a against the pushing force of the flat spring 63 to prevent the over-engagement.
In the conventional lens driving mechanism using the feed screw 60, however, since a ball is used in the over-engagement preventing arrangement, the torsion pitch of the feed screw 60 cannot be reduced. For this reason, the proportion of the deceleration by the feed screw 60 is not sufficient, so that the proportion of the deceleration by the gear train in the deceleration system between the AF motor and the feed screw 60 is far greater than that of the deceleration by the screw. Thus, this arrangement is not a satisfactory solution.